1. Field of the Invention
This invention relates to a color separation optical system for a color television camera, and more particularly to an improvement in a color separation optical system for a color television camera of relay lens type.
2. Description of the Prior Art
A color separation optical system for a color television camera of the so called relay lens type includes a relay lens system between the taking lens and the image pick-up tubes of the camera. The relay lens system divides an image of the object of the camera focused by the taking lens into three color images of red, green and blue. The three color images are then focused on the three image pick-up tubes. The relay lens system comprises a single first relay lens, color separation optical elements such as prism blocks or dichroic mirrors, and three second relay lenses provided one for each of the three color images. In the color separation optical system of the relay lens type as described above, two color images are obtained by reflection and one color image is obtained by transmission from the color separation optical elements, and it is possible to make the optical axes of the two second relay lenses for the two color images obtained by reflection parallel to the optical axis of the other second relay lens for the other color image obtained by transmission. Therefore, it is possible to orient the three color tubes in parallel to each other and accordingly to make the size of the television camera compact and avoid the influence of terrestrial magnetism of the tubes.
Further, in the above described color separation optical system of the relay lens type, the image of the object focused by the taking lens is formed on the incident side focal plane of the first relay lens so that light is collimated by the first relay lens until it reaches the second relay lenses. When the exit pupil of the taking lens is registered with the entrance pupil of the first relay lens, an image is obtained in a good condition by making the image focused by the taking lens formed on the incident side namely the front focal plane of the first relay lens. However, when the exit pupil of the taking lens is not registered with the entrance pupil of the first relay lens, the brightness of the image in the marginal portion thereof is lowered and sometimes a marginal part of the image is cut out. In order to prevent the above problem, it has been known in the art to insert a field lens having a proper power between the taking lens and the first relay lens so as to make the pupils substantially registered with each other. The above measurement utilizing a field lens has a defect in that the field lens must be very carefully handled because the field lens is positioned on the image plane of the taking lens and accordingly the scratches or dusts on the surface of the taking lens are apt to be observed in the picture of the television.
On the other hand, when the exit pupil of the taking lens is registered with the entrance pupil of the relay lens system, there is no need to insert the field lens in the optical system. However, it is disadvantageous to limit the kind of the taking lens to the one which has the exit pupil registered with the entrance pupil of the relay lens system. This is because the color television camera provided with the relay lens type color separation optical system is normally expected to have a feature that a various kinds of taking lenses can be interchanged by changing a field lens in the camera. In the conventional color television camera of the relay lens type, therefore, it is quite popular to interchange a field lens to accommodate to a variety of taking lenses. Accordingly, in this kind of the color television camera, even when the exit pupil of the taking lens is registered with the entrance pupil of the first relay lens and no field lens is necessitated, a plane parallel plate is usually inserted as a field lens of no power.
The above discussion will be described in further detail with reference to FIG. 1 hereinbelow. Referring to FIG. 1, a field lens 2 is inserted between a taking lens 1 and a first relay lens 3 of a relay lens system. The field lens 2 has a certain thickness and the first principal plane (front principal plane) A thereof is not registered with the second principal plane (rear principal plane) B, and a space H is formed therebetween. An image of the object is first focused on the front principal plane A by taking lens 1 which is then transferred to the rear principal plane B. Therefore, by making the front focal plane of the first relay lens 3 registered with the rear principal plane B of the field lens 2, the light flux behind the first relay lens 3 can be collimated and an image can be obtained in a good condition by the color separation optical system. In this case, the power of the field lens is determined according to the displacement of the exit pupil of the taking lens 1 from the entrance pupil of the relay lens system including the first relay lens 3. In order to accommodate to the various kinds of taking lenses, various kinds of field lens of different power are prepared.
When the exit pupil of the taking lens 1 is registered with the entrance pupil of the relay lens system, the power of the field lens 2 is determined to be zero which means no field lens is required. However, if the field lens is completely removed from the optical system, the image plane of the taking lens 1 is displaced from the focal plane of the first relay lens 3 by the space H as shown in FIG. 1 and accordingly the light is not collimated by the first relay lens 3. This results in failure in obtaining a color image in a good condition by the television camera.
In order to prevent this problem, the optical path length should be corrected by the space H in FIG. 1. The relay lens system including the first relay lens 3 is firmly fixed to the camera housing, and the taking lens 1 is also fixed to the camera housing, and accordingly, it is almost impossible to move these lenses 1 and 3 along the optical axis thereof to compensate for the space H. Therefore, in the prior art, a plane parallel plate having a thickness H is inserted between the taking lens 1 and the first relay lens 3 as a kind of field lens for compensating for the optical path length of the space H. Therefore, even when the field lens is not required, a plane parallel plate is used as a field lens, and accordingly, the handling of the field lens must be carefully conducted so as not to carry dusts or scratches on the surface of the field lens including the plane parallel plate.